<!DOCTYPE html>
<html lang="zh">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input:focus[required] {
            outline: none;
            border: 2px solid blue;
        }

        input::placeholder {
            color: gray;
        }
    </style>
</head>

<body>
    <form>
        <input type="text" name="username" required placeholder="Username">
        <input type="email" name="email" required placeholder="Email">
        <input type="submit" value="Submit">
    </form>
</body>

</html>